Endodontic therapy is used to save a patient's tooth when the soft tissue (pulp) in the tooth center becomes infected or nonvital. Steps in such therapy include opening the tooth, removing the pulp, cleaning, shaping, and smoothing the dentinal walls, and filling the tooth with a desired material, such as gutta percha.
In preparing a root canal for filling, it is important to consider the desired shape of the prepared root canal. The ideal root canal preparation is a continuously tapering funnel shape from the root apex to the orifice of the canal, with the narrowest dimension at the apex and the widest dimension at the orifice. Ideally, the prepared root canal should be the same shape as the original canal, except larger. Historically, the canal was prepared with files fabricated of stainless steel having a standardized taper of about 0.02 mm/mm. With these instruments, the taper of the funnel shape was created by the so-called "step back technique," involving taking progressively larger diameter instruments into the canal at serially shorter lengths (typically either 0.5 or 1.0 mm for each step back). This technique is sensitive, as well as tedious and fatiguing. A further drawback of the step-back technique is that, while it does create a funnel shape, it is not a continuous funnel, as it leaves small steps in the canal walls. These steps make filling more difficult.
Within the last few years, various companies have marketed files, both hand and rotary files, made of nickel-titanium, which is a much more flexible material than stainless steel. The rotary instruments may be used in a slow-speed, high-torque handpiece, which allows for better centering of the canal preparation and less operator fatigue. Such nickel titanium tools still need to be used in a step-back fashion, however, because the instruments become too stiff and have a tendency to screw themselves into the tooth when the taper is increased beyond approximately a 0.06 mm/mm taper.